drm/amdkfd: Fix spurious debug exception on gfx10

s_barrier triggers a debug exception when issued with PRIV=1,
DEBUG_EN=1. This causes spurious notifications to rocm-gdb.

Clear MODE before issuing s_barrier and restore MODE afterwards
in the context restore handler.

Signed-off-by: Jay Cornwall <jay.cornwall@amd.com>
Tested-by: Laurent Morichetti <laurent.morichetti@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>